1. Field of the Invention
This invention relates to an optical system for an endoscope, particularly to an optical system for use in the distal end of the endoscope.
2. Description of the Related Art
Since the inside of a body cavity when it is the observation object of a medical endoscope is dark, the inside needs to be illuminated, when it is observed. To perform the illumination using an endoscope, light exiting from a light source is generally guided to a distal end portion of the endoscope by a light guide fiber bundle disposed in the endoscope, and an observation object in the body cavity is illuminated via an illumination optical system disposed in the distal end portion to illuminate a visual field area.
If a broad area can be observed at the same time with the endoscope, the medical check-up time can be reduced to reduce a patient's burden. Therefore, the field angle of an observation optical system in an endoscope is generally set to be broad. Moreover, the illumination system (light guide fiber bundle, illumination optical system, etc.) is disposed in such a manner that the broad area can be brightly illuminated in accordance with the large field angle of the observation optical system.
Specifically, first, a light guide fiber bundle having a large numerical aperture (NA), for example, an NA of 0.6 or more is selected as the light guide fiber bundle for the endoscope, and a light beam having a large light distribution angle is guided to the distal end portion of the endoscope with a high light transmission efficiency by use of this light guide fiber bundle. Moreover, the light distribution angle is further broadened by the illumination optical system disposed in the distal end portion of the endoscope, so that the visual field area of the observation optical system can be well illuminated.
In recent years, there has been a demand for an endoscope having a larger field angle. To meet the demand, an observation optical system having a larger field angle needs to be disposed in the distal end portion of the endoscope. Moreover, the illumination optical system is required to have a large enough light distribution angle so as to be capable of sufficiently illuminating the broader visual field area of the observation optical system.
FIG. 27 is a diagram showing one example of a conventional illumination optical system for an endoscope. This illumination optical system is made up of a negative lens 2 disposed in front of an emission end surface of a light guide fiber bundle 1, and the whole visual field of the observation optical system (not shown) can be illuminated with the light emitted from the light guide fiber bundle 1. In this illumination optical system, the curvature of a concave surface of the negative lens 2 needs to be increased in order to broaden the light distribution angle. However, in this illumination optical system, the light emitted from the light guide fiber bundle 1 is outwardly refracted. Therefore, the outer diameter of the negative lens 2 needs to be increased in order to prevent the light from being eclipsed in the periphery of the negative lens 2. Accordingly, there is a need to increase the outer diameter of the distal end portion of the endoscope. Therefore, the increasing of the curvature of the concave surface of the negative lens 2 goes against a demand for the reduction of the diameter of the endoscope, and is not realistic.
FIG. 28 is a diagram showing another example of a conventional illumination optical system for an endoscope. This illumination optical system is described in Laid-Open Japanese Patent Application No. 10-239586, and can satisfy the demand for the reduction of the diameter of the endoscope.
The illumination optical system shown in FIG. 28 comprises a plano-convex lens 3 disposed in front of the emission end surface of the light guide fiber bundle 1 and having a positive power. The illumination optical system for the endoscope brings the light emitted from the light guide fiber bundle 1 into convergence, and then allows the light to diverge. Therefore, it is possible to reduce the outer diameter of the lens in the illumination optical system as compared with the illumination optical system using the negative lens 2 shown in FIG. 27.
Additionally, the light guide fiber bundle 1 is formed by bundling a plurality of fibers. As shown in FIG. 29A, only core portions C of each fiber transmit the light. Therefore, in the emission end surface of the light guide fiber bundle 1, only the core portions arranged in a dot-matrix form emit the light. As shown in FIG. 29B, the illumination optical system having a positive power projects the emission end surface of the light guide fiber bundle 1 onto an object surface 4 in a magnified manner. Therefore, as shown in FIG. 29C, the illumination light projected onto the object surface 4 causes a dot-matrix-like illuminance unevenness (non-uniformity of illuminance).
To solve this problem, an illumination optical system is described in Laid-Open Japanese Patent Application No. 5-157967 or 6-148519 in which a single fiber rod is inserted between the light guide fiber bundle 1 and a positive lens to prevent the dot-matrix-like illuminance unevenness from being generated. FIG. 36 is a diagram showing one of the described examples.
In the illumination optical system shown in FIG. 36, the length of a single fiber rod 5 needs to be sufficiently increased in order to obtain from only the single fiber rod 5 a sufficient light diffusing effect for preventing the generation of the dot-matrix-like illuminance unevenness. However, when the single fiber rod 5 is lengthened, the total length of the illumination optical system 6 increases, and this cannot satisfy a demand for the reduction of the total length of the optical system in the endoscope.
FIG. 30 is a diagram showing another example of a conventional illumination optical system for an endoscope. This illumination optical system for an endoscope is described in Laid-Open Japanese Patent Application No. 2002-182126.
In the illumination optical system 5, the single fiber rod is used as the material of a third lens 53 in order to eliminate the dot-matrix-like illuminance unevenness, and the curvature is imparted to one surface of each lens. Since three lenses are included, the total length of the optical system necessarily increases.
The above-described illumination optical system is usually used in combination with an observation optical system having a field angle of 120° to 140°, but when the illumination optical system is used in combination with the observation optical system having a field angle of 150° or more, there occurs a disadvantage in that the visual field periphery (vicinity of an inner edge of an observable area (visual field)) of the observation optical system darkens, and cannot practically be observed.
This aspect will be described hereinafter from two viewpoints: light distribution characteristics of the illumination optical system; and a positional relation between the observation optical system and the illumination optical system.
First, the light distribution characteristics of the illumination optical system will be described.
An inner surface of the stomach or large intestine, which is an object of a medical endoscope, can be regarded as a schematically spherical surface or an inner surface of an empty tube. In these two surfaces, the spherical object surface is more difficult to be brightly illuminated in a broader area than the inner surface of the empty tube. Therefore, the illumination optical system that exhibits satisfactory light distribution characteristics when evaluated under conditions where a spherical object is illuminated, also exhibited a satisfactory light distribution characteristics, when used for illuminating another object.
In an endoscope having a field angle of 140° or less, the illumination optical system preferably has such spherical light distribution characteristics (illuminance distribution of the illumination light on the spherical object surface) as to be as flat as possible from a center of the visual field to the periphery and to reduce the illuminance unevenness.
FIG. 31 is a diagram showing one example of the spherical light distribution characteristics of the negative lens shown in FIG. 27. It is seen that the amount of a change of the illuminance is small up to an emission angle of 40°, but the change amount of the illuminance increases, when the angle of emission exceeds 50°. The angle of emission at which the change of the illuminance increases corresponds to a peripheral portion of the visual field on the object surface. Therefore, one feels that the periphery of the visual field is dark as compared with the vicinity of the center. The illumination optical system having such spherical light distribution characteristics has practicability as an illumination optical system which illuminates the visual field of an observation optical system having a field angle of 100°. However, it can be confirmed through experiments that one feels that the illumination optical system is unsuitably dark for the system which illuminates the visual field of the observation optical system whose angle of field exceeds 100°. When the curvature of a curved surface of this negative lens is increased, the light diverges more intensely. Therefore, the light distribution angle of the illumination optical system broadens, and it is possible to increase the angle of emission at which the change of the illuminance increases. However, the outer diameter of the negative lens increases as described above. This illumination system empirically bears a practical use in an endoscope whose angle of field is 120° or less. When this angle is exceeded, the visual field periphery cannot be observed.
Next, FIG. 33 shows the spherical light distribution characteristics of an illumination optical system made up of three positive lenses shown in FIG. 30. In the figure, notation “example” indicates that the corresponding curve shows the characteristics of the example of this prior-art document. The light guide fiber bundle has an NA of 0.76. The illumination optical system retains a high illuminance at an emission angle of about 50°. Therefore, when the system is used in an endoscope having a field angle of about 120°, the brightness is sufficient up to the visual field periphery, and a flat light distribution is obtained up to the visual field periphery. However, when the angle of emission exceeds 55°, the illuminance rapidly drops, and darkness becomes conspicuous. Therefore, although the illumination system has a high absolute illuminance, it is unsuitable in a system for illuminating the visual field of an observation optical system having a field angle of 150° or more, since the change of the illuminance is large, thereby giving an impression that the visual field periphery is dark. Supposing that it is possible to realize an illumination optical system having a high illuminance up to an emission angle of 70°, an area of a field angle of 150° is sufficiently brightly illuminated. However, when the system has the above-described light distribution characteristics, even a non-observed area outside the visual field area is illuminated, and this is not desirable with regard to illumination efficiency.
To realize a wide-angle illumination system, the change of the illuminance is comparatively moderate, and the absolute illuminance needs to have a value which is not less than a certain value in the visual field periphery, but the above-described illumination optical system does not satisfy this requirement.
Next, the positional relation between the observation optical system and the illumination optical system will be described. When a distal end surface of the endoscope comes within 10 mm or closer to the object, the light distribution in the visual field of the observation optical system largely depends on not only the light distribution characteristics of the illumination optical system but also on the positional relation between the observation optical system and each illumination system. FIG. 34 is a diagram showing a positional relation between the illumination optical system and the observation optical system, and that between an area illuminated by the illumination optical system and the visual field of the observation optical system. In the figure, reference numeral 10 denotes an illumination optical system, 9 denotes an observation optical system, an area 11 where slant lines are drawn denotes an area illuminated by the illumination optical system 10, and 12 denotes a visual field of the observation optical system. In the case where the distal end surface of the endoscope comes close to the object, when the observation optical system is apart from the illumination optical system, as shown in FIG. 34, there are problems in that the area 11 illuminated by the illumination optical system 10 does not cover the whole visual field 12 of the observation optical system 9, and that a portion which is not included in a slant line portion in the visual field 12 is not bright. The conventional illumination optical system for an endoscope which is to improve the problems is described in Laid-Open Japanese Patent Application No. 2001-166223.
FIG. 35 is a diagram showing a layout of the observation optical system and the illumination optical system in the endoscope described in the Laid-Open Japanese Patent Application No. 2001-166223. This endoscope is provided with four illumination optical systems around the observation optical system 9, and the four illumination optical systems include two types of illumination optical systems 101 and 102 having different quantities of emitted light. The Laid-Open Japanese Patent Application No. 2001-166223, describes that, by use of this arrangement, a balance of the light distribution is improved in the case where the distal end surface of the endoscope comes close to the object.
However, it is difficult to improve the balance of the light distribution at the time when the distal end surface of the endoscope comes close to the object up to a sufficiently satisfactory level of wide-angle illumination only by the positional relation between the observation optical system and the illumination optical system and the quantity of emitted light. That is, when the improvement of the balance of the light distribution is considered, the light distribution characteristics inherent in the illumination optical system need to be sufficiently considered.